U.S. Pat. No. 3,957,354 describes a diffractive subtractive color filtering technique whereby color information is recorded as a square wave grating surface relief pattern in a photoresist layer of a given thickness which corresponds to the amplitude of a given primary color. By recording separately for the three primary colors in the proper thickness for each and placing the recordings in series and proper registration with each other, a full color picture can be derived.
Thus three photoresist recordings are made from which metal stampers can be prepared to provide masters from which a replica, e.g., a transparent thermoplastic sheet, can be prepared with the surface relief patterns embossed in it. The complete color picture is reconstructed by sandwiching the three subtractive primary color components, i.e., each having a surface relief pattern of a different depth. The depth dimensions of any of these relief patterns is on the order of 2 micrometers or less.
A more compact method of recording is described in U.S. Pat. No. 4,082,453, filed June 9, 1976, incorporated herein by reference, whereby two of the three color components can be recorded together. A layer of photoresist is chosen so that, after development, unexposed regions of photoresist will be equal in thickness to the sum of the amplitudes of two superposed two-level diffraction structures. The resultant surface relief pattern thus has a stepwise configuration with multi-levels. The third color is recorded separately as before. The final embossed sheet can be embossed on one side with the multi level pattern and on the other side with the third pattern, thus obviating the need for sandwiching two or more embossed sheets together.
Copending application of Gale et al, Ser. No. 705,931 filed July 17, 1976, now abandoned, discloses a method whereby all colors can be recorded in a single surface relief pattern. According to this method, a surface relief pattern in a first desired depth is recorded in photoresist on a substrate and developed, a layer of metal or a silicon oxide is evaporated onto the photoresist pattern and the photoresist removed, leaving a silicon oxide pattern of that depth on the substrate. A second layer of photoresist is applied which will provide a depth for the second color pattern, the photoresist exposed and developed to form a bilevel pattern superposed on the first pattern; a second evaporated layer is applied and the photoresist removed. By repeating these steps, multilevel patterns of the evaporated layer are formed on the substrate. Since the evaporated layers are hard and durable, they can be used directly as a master for embossing the multilevel pattern into thermoplastic films.
Alternatively, a multi-level pattern can be made by subtracting material from the substrate, as by sputter etching or plasma etching the photoresist pattern. After developing the first photoresist pattern, the exposed substrate and photoresist surfaces are etched away to the first pattern depth corresponding to one primary color. The photoresist layer thickness is chosen so that some photoresist remains when the first depth in the substrate is obtained. The photoresist is removed when etching is terminated. A second layer of photoresist is then applied, exposed and developed and a second etching step carried out to etch to the second pattern depth corresponding to another primary color. These steps are repeated for the third pattern depth corresponding to the third primary color.
It will be appreciated that the quality of the reconstructed color picture depends on the squareness and the ratio of groove width to line width of the individual square wave gratings in the multi-level relief pattern. At pattern periodicities of 5 microns or more, evaporation methods and sputter etching or plasma etching methods work well. However, problems are encountered with patterns of less than 5 microns periodicity.
In the additive method, evaporated coatings tend to assume a trapezoidal, rather than a square shape, probably due to a shadowing effect, unless great care is taken to avoid this.
Conventional sputter etching of a photoresist layer on a substrate using argon as the sputtering gas, tends to produce a slant walled pattern, probably due to redeposition of sputtered material on the substrate because of collisional scattering in the gas phase.
Conventional plasma or chemical etching of a fine photoresist layer on a substrate produces severe undercutting, which causes problems during replication.
An improved method for forming straight walled square wave surface relief patterns of micron dimensions would be highly desirable for this application.